When it was discovered that fiber optic cable could be used more efficiently for the transmission of telephone conversations, an enormous replacement undertaking became apparent. The copper cable that was previously used underground had to be replaced by fiber optic cable in a costefficient manner which would maintain the integrity of the transmission cable.
When installing the fiber optic transmission cable, which is usually sheathed in a thermoplastic insulator, there are a number of problems that must be avoided. The plastic sheathing itself must not encounter any sharp surfaces that damage or shave it away to any substantial degree. If this is done, exposure of the light guide occurs, along with the pertinent problems. Moreover, tension cannot exceed a certain safety margin of the transmission cable or it may break.
Because of the forces necessary to pull the transmission cable through a duct, it is highly advantageous to lower the friction between the transmission cable and its duct. When the coefficient of friction of the duct is lowered, the resulting lower forces to pull the cable through the duct allow longer lengths to be strung without a relay.
Previously, transmission cable was pulled through a duct which may have had either no interior ribs or longitudinal interior ribs. An example of the ribbed duct is shown in U.S. Pat. No. 4,565,351, which is hereby incorporated by reference. Alternately, transmission cable was strung through corrugated duct, i.e., transverse ribs. Each of these ducts caused problems. Smooth wall duct had the highest coefficient of friction, and therefore required high forces to be used when pulling a transmission cable therethrough. As a result, relatively short lengths of transmission cable could be placed underground before the forces built up to the point where the cable itself became endangered.
Corrugated and longitudinally ribbed duct reduced surface friction with the cable. Both the longitudinally ribbed and smooth wall ducts, however, had a tendency to damage the cable jacket. Such damage to the jacket would not normally be discovered until a later date, and replacement of the cable would be necessary. The replacement process was highly undesirable and needlessly expensive.
Corrugated tubing had a relatively low coefficient of friction relative to the transmission cable, and did not have a tendency to damage the sheathing. However, corrugated duct is relatively thin and has uneven walls because of the necessity of continuously blow-molding it during its manufacturing process. Moreover, the thin walled corrugations had a tendency to stretch or break during field installation because of their own relatively low tensile strength. The flexible nature of corrugated tubing also allowed it to rotate and shear. The thin wall thickness also wore quickly and perforated when transmission cable was pulled through it. The depth of the corrugations is generally two to four times as deep as longitudinal ribs, thus causing the lubricant to well up between successive corrugations and impeding normal cable lubrication efforts.
This invention uses a duct including a polymeric tubing having an inner and outer wall. The inner wall has spiral ribs. The use of spiral ribs has been found to give the best combination of results when used with a transmission cable for reducing the friction between the cable and the duct, providing structural integrity of the tubing, and not damaging the sheath on the cable. The spiral ribs act much like corrugations in that they do not shave off the sheathing because the cable tends to pass over them in a transverse manner. The internally spiraled duct is much stronger than corrugated tubing and does not have a tendency to rip or tear during its installation. It is also flexible enough to go around corners. The valleys between the spiral ribs also act as a reservoir for lubricant which is commonly used to reduce friction between the cable and the duct.